The causes of the conflict were both religious and diplomatic, and resulted in Spain's failed attempt to invade England with the Spanish Armada.
Philip II of Spain was a devout Catholic and devoted to maintaining Catholicism in Europe. He had been married to Elizabeth I's half sister, Mary I during her brief reign. Since Mary was Catholic, Philip believed that her reign marked the return of England to the Catholic Church. When Mary died and Elizabeth ascended the throne, Philip proposed marriage to her also. Elizabeth rather coyly avoided a flat response, and thus strung Philip along for some time. When it became apparent that Elizabeth would not marry him, let alone convert to Catholicism, Philip supported a number of plots to have Elizabeth overthrown and/or assassinated. The most famous of these plots involved Mary Queen of Scots, Elizabeth's Catholic cousin who had been forced to flee Scotland. After Mary was implicated in two plots to assassinate Elizabeth and take the throne for herself, Elizabeth had no choice but to order Elizabeth's execution. This action enraged Philip.
A second cause was the actions of Francis Drake, the English privateer who attacked Spanish shipments and settlements in the New World. When Drake found his way across the Atlantic blocked by Spanish ships, he sailed west and circumnavigated the globe to return to England. Philip demanded that Elizabeth surrender Drake to him so that he might stand trial in Spain for his actions. Rather than surrender him, Elizabeth knighted Drake on the deck of his ship when he docked. Philip was further outraged, and sent the Spanish Armada to retrieve troops from the Netherlands and invade England. The plan again was to remove Elizabeth. At the time the Armada was launched, Elizabeth met with her troops dressed in full battle armor and told them
I know I have but the body of a weak and feeble woman, but I have the heart and stomach of a King; and of a King of England too.
The failure of the Armada doomed any chances of England returning to Catholicism.